Global Mapper v25.0

Differences in ArcGIS (10.2) and Global mapper (v14) for the same shapefiles

Hi there,
I have a question and hope someone can answer this. When I want to display two shapefiles in both programs, they are displayed differently. This might have to do with the projections of these files: One of the two uses a azimuthal equidistant projection, whereas the other not, but both use the GCS_WGS_84. So in my case I have a point location and an area shapefile, the point is inside the area in ArcGIS and outside the area in GM. What could cause these two different views and which one is the correct one? Note: I am not asked to enter information by either of the program, I just drag them in and they are displayed, as I have projection files as well.

Thanks,
Angela

Answers

  • tjhb
    tjhb Global Mapper User Trusted User
    I would guess that one or more of the projection files are incomplete or incorrect.

    You could try loading both shapefiles without the projection files, assigning the correct projections manually.
  • Thanks for the reply - I already did that. The question is still why one program reacts differently from the other in the first place? I still don't understand why both of them won't complain about the projection files, but display it different.
  • tjhb
    tjhb Global Mapper User Trusted User
    edited August 2015
    The question is still why one program reacts differently from the other in the first place?
    If the projection file(s) were incorrect or incomplete (as is very often the case for shapefiles, since the PRJ format itself is incomplete--you're often better off without them), then you could not expect different software to interpret them in the same way. "From a contradiction anything follows." All bets would be off.

    But if you have already opened all files without any projection files, and manually assigned the correct projections in both ArcMap/ArcCatalog and Global Mapper, and got the same result, then that does not arise.

    To get further you probably need to give more complete details: all of the projection parameters for both shapefiles, a full file list, all steps to load both datasets in each software environment, possibly screenshots of the projection dialogs and the result, ideally some sample data. I.e. pedantry.
  • bmg_bob
    bmg_bob Global Mapper Programmer
    I recommend getting the latest release of Global Mapper and try loading your data. If the problem persists in the latest release, then your best bet is to send an e-mail describing your workflow, your sample data, and the results to Blue Marble Geographics support (geohelp@bluemarblegeo.com). This will ensure that the support team sees the problem and can determine the solution. Thank you.

    As far as whether ArcMap or Global Mapper is correct: don't you know whether or not the point is supposed to be inside the area? You have to tell us which way is correct.
  • Thank you both - When I input the shapefiles without any prj file into GM v14 and v16.2. Then assign the projection which I take from the prj files manually (different for both), then I still have the case as before. And I don't know whether the point location is inside or outside the area, because the area is a downloaded shapefile from a website. ;-) In any case I will write an email to the support group and outline the whole approach. Thanks again.
  • tjhb
    tjhb Global Mapper User Trusted User
    edited August 2015
    Ideally the data provider will have given full projection information somewhere in their metadata, not just PRJ files (which don't capture everything for all projections). So be sure to include that too, if you can find it(!)

    Once you get a resolution you might consider letting the provider know about the difficulty with correct interpretation of their data. (If they're a publicly funded outfit and you can't find full metadata you could try asking them what metadata standard they currently adhere to.)
  • tjhb
    tjhb Global Mapper User Trusted User
    By the way if I had to guess (blind) I would guess that one of the shapefiles does *not* have WGS84 as its datum, but something closely related. (Meaning perhaps that one of the PRJ files is wrong.)
  • Jake
    Jake Global Mapper User Trusted User
    Angela, This might be too late for you but in case not here it goes. Others may find this helpful.

    A straight line means different things to different software and they can take different paths to get to the next vertex. The first thing you need to answer is, is ArcGIS drawing geodesics? Is GM drawing geodesics? If the answer is not the same then you will have an issue with topology. Think of it like taking two roads to get to the same destination. You may be aware already but a geodesic is the shortest path between two points. A straight line on a map projection will not be the shortest distance.

    So if there was a projection specified, then the software *may* be using a straight line (path) on the map projection (grid). If no projection was specified (i.e. geographic aka plate carree) then a 3rd path will be drawn. This third path is really just a variation of the second case but most people are not aware of the consequences of working with geographic data so it bears mentioning. Also, you will often find settings to control how each software handles paths.

    If you are unsure of the software you can draw a lat/long box (LL 0,0 UR 30,30) in both applications and see if they report the same geometry (area, length, perimeter etc).